The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for optimizing an electrical response to a set of conductive layers on a substrate.
In the processing of a substrate, e.g., a semiconductor wafer, MEMS device, or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
Metals are particularly important materials in substrate manufacturing. For example, in a manufacturing method, known as dual damascene, dielectric layers are electrically connected by a conductive plug filling a via hole. Generally, an opening is formed in a dielectric layer, usually lined with a TaN or TiN barrier, and then subsequently filled with other conductive material (e.g., aluminum (Al), copper (Cu), tungsten (W), etc.) that allows electrical contact between two sets of conductive patterns. This establishes electrical contact between two active regions on the substrate, such as a source/drain region. Excess conductive material on the surface of the dielectric layer is typically removed by chemical mechanical polishing (CMP). A blanket layer of silicon nitride or silicon carbide may then be deposited to cap the copper.
Subsequently, in order to insure that the process is within acceptable parameters, it is often important to determine the electrical film/layer properties (e.g., thickness, sheet resistance, etc.) of a conductive layer at a particular point on the substrate. One method of measurement is the use of eddy current sensors. Generally, eddy currents are currents that are induced in a conductive media by an alternating magnetic field.
In general, if a first alternating current is applied to a wire wrapped in a generally solenoidal shape (e.g., the wire in an eddy current sensor), a first alternating electromagnetic field forms in and around the solenoid extending beyond the ends of the solenoid a distance on the order of the diameter of the solenoid. If this first field is brought into proximity with a second conductor (e.g., a conductive layer on the substrate) a second alternating electrical current will also flow in the second conductor, causing a second field that interacts with (e.g., adds vectorally to) the first field and results in a perturbation to the field around the probe. These perturbations in the probe's initial field may cause detectable changes in the probe's electrical characteristics including the probe's impedance and frequency response. Using an impedance-voltage converter, the impedance change can be converted into a voltage change for further signal processing and analysis.
Many techniques are available for producing a signal from these detected differences in eddy current probe characteristics. For example, in a first technique, the width of the frequency dependent power absorption of the probe/eddy current sensor system (sensor system) can be reported. Likewise, in a second technique, the change in the magnitudes of the real and/or imaginary parts of the probe impedance can be reported between the probe and the second conductor. These measurements are generally made using passive or active circuitry to produce a range of voltages that can be bounded by the signal with no second conductor present and the signal with a second conductor causing maximal change in the signal. The exact shape, thickness and conductivity of the second conductor that causes the maximal change in the probe signal generally depends on the probe geometry, excitation frequency and the method adopted for measurement, but generally it is a thick (on the order of many times the diameter of the probe) conductive film (layer) placed as near to the probe as possible.
Depending on the application, conductive or magnetic elements can also be incorporated into the design of the probe in order to modify the spatial extent and magnitude of the probe field and hence the spatial and electrical sensitivity to the second conductive layer. For optimum performance, the sensor system should maximize sensor system sensitivity to the desired electrical property of the conductive film (e.g., thickness, sheet resistance, etc.) while minimizing the sensor system's sensitivity to all other effects and variables.
Generally, the electrical response of sensor to the magnetic field (eddy current perturbations), and hence its accuracy, is affected by the proximity (substrate proximity response) of the sensor to the substrate. That is, as the exciting probe field is of limited spatial extent and its magnitude decreases as the position increases from the probe, the overall eddy current perturbations caused by a second conductor being measured also decrease as the second conductor is moved further from the probe. Thus, an eddy current sensor may be sensitive to both proximity and electrical film properties. In general, it is difficult to isolate the portion of the electrical response caused by the set of electrical film properties (electrical film property response) from the portion of the electrical response caused by proximity (substrate proximity response), which may subsequently introduce an error in the reported value.
In addition, the set of electrical film properties for a particular substrate may itself be variable. For example, the electrical response of sensor may be affected by the resistivity of the conductive film. That is, eddy current signal variation is primarily proportional to the inverse of film resistivity. Electrical resistivity (also known as specific electrical resistance) generally indicates how strongly a material opposes the flow of electric current. A low resistivity generally indicates a material that readily allows the movement of electrons. However, resistivity is also generally dependent on temperature.
Referring now to FIG. 1, a simplified diagram of an eddy current sensor is shown. Generally, changes in the sensor's coil impedance 102 are caused by varying the distance 104 between the sensor (coil) and substrate 106. Since the electrical parameters of target material resistivity and permeability may determine the magnitude of the measured sensor perturbation, the sensor system is generally calibrated for the target material.
One solution to improve the response of a given sensor may be to average out the proximity errors of multiple sensors, each concurrently trying to measure the same point on the substrate from the same proximity (e.g., concurrent multiple sensors). For example, two sensors, each with a known and fixed proximity to each other, may be positioned at a fixed proximity to a conductive layer positioned between them. In a common implementation, one sensor is positioned above the substrate and the other sensor is positioned below the substrate. If each sensor has a substantially identical sensitivity to proximity, the electrical response on any one sensor may be substantially equal but opposite to the electrical response on the other sensor. Subsequently, averaging together a signal from each sensor may result in a combined signal that is much less sensitive to the position (proximity) of the conductive layer to either one of the two sensors, and which subsequently may be used to better report the desired electrical property of the conductive film (e.g., more independent of proximity).
By periodically calibrating the sensor system (sensors, substrate geometry and substrate handling, stage movements, etc.) prior to making measurements, the proximity error in theory may be cancelled out by averaging a pair of measurements taken when the substrate is placed in the known position between the sensors. In practice, however, it is often very difficult to repeatably and precisely position the eddy current sensors with respect to the measured conductive layer.
For example, the equipment used to position a substrate between sensors may have a tolerance range that is too broad, so that the perturbations of the sensors due to changes in the substrate film thickness are substantially similar when compared to the sensor perturbations measured due the differing proximities at different measurement placements or times. Likewise, a mechanism used to move the substrate with respect to the sensors (i.e., turntable, etc.) may induce vibrations in the substrate or changes in the substrate proximity with amplitudes that cause perturbations in probe signals that exceed the measured differences in film thickness or introduce uncertainty in the reported film thickness in excess of the desired precision for the sensor system. Subsequently, even relatively small proximity variations may introduce substantial errors in the measurements, presenting a problem for high precision measurements, such as substrate manufacturing.
In addition, even if the proximity error for concurrent multiple sensors could be substantially minimized, it may still be desirable to make the measurements at different points in time (e.g., sequential measurement). For example, since sensors are often located on a sensor swing arm, it may be inconvenient to align both sensors when moving the sensor swing arm across the surface of the substrate. That is, two sensors may be placed on the sensor swing arm such that they form a line parallel to a vector that is tangent to the rotation of the substrate on a turntable. As the sensor arm swings across the rotating substrate, the angle between the sensor line and the tangent vector may increase to the point at which both sensors cannot be positioned over the same point on the substrate at the same time. Additionally, the sensor swing arm construction itself may prevent locating the sensors on top of each other, or interference from one sensor (e.g., cross talk) may prevent the simultaneous use of both sensors.
Referring now to FIG. 2, a simplified diagram of a substrate in a mechanism to rotate it with a sensor arm is shown. In this example, substrate 202 rotates in direction 208, as sensor swing arm 204 moves sensors 206 across the surface of substrate 202.
In view of the foregoing, there are desired methods and apparatus for optimizing an electrical response to a set of conductive layers on a substrate.